20mm Hispano AP Ammunition

With the Hispano 20mm cannon becoming a more common armament for the Royal Air Force in 1940 the question of using the cannons to attack AFVs was soon raised, bombing at the time were too inaccurate to deal with tanks and tank armour had progressed to a stage were machineguns were useless. The Hispano was initially armed with High Explosive and Ball ammunition, Ball did not have the penetration to deal with tanks so development of an armour piercing shot went ahead. Initial attempts failed and with urgent requests for 20mm AP ammunition the failed attempt was modified so that only the tip of the projectile was hardened, 5000 of these were dispatched to Egypt towards the end of the year and had penetration around 5mm better than that of Ball.

By the 5th of May 1941 three types of 20mm AP ammunition were available.

One-piece tungsten-steel, hardened at tip only (Could only be used in steel-ended magazine type).

Two-piece design - chrome-molydenum steel body with moulded-on bakelite ballistic cap (Could be used in all types of feed).

It was considered that the first two were only stop-gaps, a contract for 100,000 rounds was place with I.C.I. The third type was considered to be the one which would finally be the approved design. Disappointing results with the bakelite ballistic cap meant the O.B. requested Messrs. Hadfield's to design a one-piece flat-nosed AP shot following large-calibre practice rather than an AP bullet following small arms ammunition practice, it was felt this would give penetration better than previous types.

Penetration against High Hardness plate, Ball ammunition was expected to penetrate 14,13mm at 200 yards 0 and 20 degrees.

Type

Striking Velocity 200 Yards (fps)

Penetration at 200 yards (0 degrees)

Penetration at 200 yards (20 degrees)

Penetration at 200 yards (40 degrees)

Striking Velocity 400 Yards (fps)

Penetration at 400 yards (0 degrees)

Penetration at 400 yards (20 degrees)

Penetration at 400 yards (40 degrees)

One-piece (all hardened)

2325

25.3

23.1

19.5

1994

22.5

18.4

14.5

Two-piece

2310

19.5

9.6

9.6

1973

26.3

9.6

9.6

Two-piece (3)

2300

24.3

16.1

10.7

It was realised that the new Hadfields design would probably not be able to be in production for at least 5-6 months so it was decided to go ahead with the bakelite ballistic cap shot as it could be used in all guns and AP ammunition for both attack of AFVs and for air-to-air use was urgently required. Performance of the ammunition was the following...

Type

Penetration at 200 yards (0 degrees)

Penetration at 200 yards (20 degrees)

Penetration at 200 yards (40 degrees)

Best One-piece (hardened tip only)

26.4

18.1

12

Two-piece(3)

27.4

24

15.7

In June 1941 requirements of AP ammunition were to be 1% of total 20mm ammunition production, this was calculated with a loading of 80% AP/20% Tracer for attack against AFVs. The air-to-air requirement was not definite until the introduction of heavy armour in German aircraft. The One-piece design became known as A.P. Mark I and the Two-piece design became known as A.P. Mark II.

20mm Hispano A.P. Mk II

Weight

140 grams

Muzzle Velocity

2660fps

Charge

Nitro cellulose powder

Identification

Black body, white tip

Penetration of Hispano ammunition July 1942 against High Hardness plate.

Type

Penetration at 200 yards (0 degrees)

Penetration at 200 yards (20 degrees)

Penetration at 200 yards (40 degrees)

Penetration at 400 yards (0 degrees)

Penetration at 400 yards (20 degrees)

Penetration at 400 yards (40 degrees)

A.P. Mark IIZ

27

24

19

24

19

15

A.P/I. Mk.IZ

20

19

15

-

-

-

Ball Mk IZ

12

11

9

10

9

7

H.E/I. Mk IZ

8

-

-

-

-

-

A.P. Mk III development

In 1941 and 1942 British units in North Africa had suffered badly against German armoured vehicles which had 30mm thick side armour, R.A.F fighters armed with the current AP ammunition could do little against these vehicles at combat ranges, attempts to meet this requirement included the 40mm Vickers S Guns and hyper-velocity AP projectiles in existing guns (littlejohn). The 40mm gun was used in service with good success but units equipped could only be used against specialised targets when general purpose fighters were badly needed. The littlejohn project did not meet with much success due to gun function difficulties with the light projectile.

So, it was decided in June 1942 to develop a composite rigid projectile capable of defeating the armour of German tanks at all combat ranges and be designed to fire from standard service fighter equipment. The design consisted of a duralumin envelope with a tungsten carbide core weighting 57.5 grams, the total weight of the projectile was around 96 grams. Penetration and ballistic trials were carried out in January 1943 and these left the stability of the projectiles in doubt but penetration was very good. Several modifications were made to improve the stability of the rounds, in May 1943 a trial was carried out against a Panzer Mk IIIH with wooden mock-ups for crew, the results were impressive as the crew were shot to pieces but the poor accuracy of the shot was still noticed.

Production of Mk III AP began in May 1943

A.P. Mk III Performance

Type

Muzzle Velocity

Striking Velocity 200 Yards with 350pfs AC speed

Penetration at 200 yards (0 degrees)

Penetration at 200 yards (30 degrees)

Striking Velocity 400 Yards with 350pfs AC speed

Penetration at 400 yards (0 degrees)

Penetration at 400 yards (30 degrees)

Striking Velocity 600 Yards with 350pfs AC speed

Penetration at 600 yards (0 degrees)

Penetration at 600 yards (30 degrees)

A.P. Mk III

3230

3200

67

48

2850

51

38

2500

39

30

At the beginning of 1944 AP. Mk IV was introduced, I haven't been able to come across the particulars of this ammunition yet but it seems to be regular AP and not APCR. Developments in 1945 included both APC and APCBC types.